1. Field of the Invention
This invention relates generally to non-contacting end face seals and more specifically to seals which can accommodate high pressure differentials between the outer and inner diameters of such seals.
2. Background Art
Mechanical end face seals of the non-contacting type have become high technology items utilizable in a variety of industries. These type of seals are designed with a great deal of care and attention to the materials, shapes, dimensions and tolerances of the component parts. Such attention to detail in the design is necessary in order to accommodate a great number of characteristics, any of which characteristics is liable to affect the efficiency or operation of seals of this type. Minor changes or alterations to any one of about 10 physical features of a seal, its components or its sealing environment may, and in most cases will, result in a change in the seals characteristics, sealing capability, wear, endurance and/or integrity. In most cases, it is desirable for the seals of this type to operate maintenance and trouble free for extensive periods of time, on the order of years. The seals are used in machinery which cannot be shut down for long without severely impairing the operation and efficiency of, for example, a large chemical plant or refinery.
Seals of this type may at times be utilized in dual or double seals, the two seals being spaced apart axially along a shaft passing through an aperture in a housing. Double seals generally also include a buffer fluid in the intermediate chamber defined by the housing and enclosed by the two seals. Various arrangements of double seals are described in commonly owned U.S. Pat. Nos. 4,290,611 and 5,375,853, both of which are hereby incorporated by reference.
U.S. Pat. No. 5,375,853 in particular illustrates and describes a dual non-contacting type seal having a relatively inert gas, such as nitrogen, as the buffer fluid at a high pressure in excess of the pressure of the sealed liquid within the housing.
It has been found that seals of the type described and illustrated in U.S. Pat. No. 5,375,853 operate well under most conditions. However, because the seal arrangements made in accordance with U.S. Pat. No. 5,375,853 were developed for specific pressure conditions, limitations arise in the event the process fluid or buffer gas pressures increase beyond the rated specification pressures of the seal. Keeping other factors the same, seals which are subjected to pressure differentials of over 200 psi between the inner and outer diameters of the seal rings encounters complications which affect the sealing capability. It has been noted that at pressure differentials exceeding 200-250 psi, there is noticeable angular rotation of the seal rings, when viewed in cross-section, around a point known as a centroid. Although the rotation is in fact more of an alteration of the seal ring shape from cylindrical to a conical configuration, the phenomenon is more easily considered as the seal rings are illustrated in cross-section, with the ring cross-section in effect actually being indicated as rotating about its centroid. For ease in illustration and description herein, this convention will be adhered to in the following description and illustrations.
It has been recognized that uneven heating may result in thermal distortion of a seal ring and that high pressure differentials between the inboard and outboard diameters of the seal rings may result in pressure distortions. Such distortions are undesirable because they cause the normally flat opposed mating seal faces of the rings to diverge from the normal sealing engagement between the seal ring faces. In a non-contacting seal, distortion of the seal rings normally causes the outer diameters of each seal ring face to rotate about the centroid toward the centerline of the seal ring, causing the seal gap to become wider at the inner diameter of the seal ring interface and to narrow at the outer diameter. This distortion in and change in axial depth of the seal ring gap is not conducive to sealing capability, because the effects of the spiral grooves pumping a gas against a dam are dissipated if the dam is not adjacent the opposed mating sealing face. The narrowing of the gap at the outer diameter is undesirable for a non-contacting seal because a smaller gap renders the seal faces susceptible to undesirable contact, resulting in premature wear of the seal faces.
One major benefit of non-contact seals is that seal ring wear is essentially kept to a minimum, occurring ideally only during start up or shut down of shaft rotation. Contact is also undesirable because frictional heat resulting from such contact causes uneven heat distribution in the seal rings, and thus in thermal distortion of the rings. To avoid excessive contact at the outer diameters of the seal interface, the primary seal ring of seals made according to U.S. Pat. No. 5,375,853 may include a shoulder at the outer diameter. Another method of avoiding seal contact at the outer diameter of a seal is described in commonly assigned U.S. Pat. No. 3,499,653, which utilizes a convex face of the primary seal ring.
Commonly assigned U.S. Pat. No. 4,407,512 describes a seal construction in which specific parameters of the seal, such as aspect ratio, semi-circular shallow recesses (hydropads) at the radially outer portion of the seal face and a net negative moment about the centroid when the seal is in operation, are used to accommodate high pressure at the outer diameter of the seal. However, the seal described in that patent is a contacting seal, so that heat generated by frictional contact of the seal faces causes thermal distortions. The seal is designed so that these thermal distortions compensate to a degree the distortions which arise from the high differential pressures across the seal face.
In the context of a non-contacting type seal, commonly assigned U.S. Pat. No. 3,804,424 describes a gas seal having thermal and pressure distortion compensation. The seal relies upon a number of orifices passing through one of the rings to provide an increase in pressure in the seal interface so that the pressure differential does not drop off, the orifices being in communication between the high pressure fluid being sealed at the outer diameter of the seal and a chamber between an inner and outer diameter dam of the primary ring.